This is paraphrased from part of one of my essays...
What the Kingdom of God in the Bible is usually about is well summarised by Ridderbos:
... the kingdom manifests itself in all sorts of ways in the person and deeds of Christ. It appears palpably and visibly in the casting out of demons (cf. Lk. 11:20) and generally in Jesus miraculous power. In the healing of those who are demon-possessed it becomes evident that Jesus has invaded the house of ‘the strong man’, has bound him fast and so is in a position to plunder his goods (Mt. 12:29). The kingdom of heaven breaks into the domain of the evil one. The power of Satan is broken. Jesus sees him fall like lightning from heaven. He possesses and bestows power to trample on the dominion of the enemy. Nothing can be impossible for those who go forth into the world, invested with Jesus’ power, as witnesses of the kingdom (Lk. 10:18f.). The whole of Jesus’ miraculous activity is the proof of the coming of the kingdom. What many prophets and righteous men desired in vain to see—the breaking in of the great epoch of salvation—the disciples can now see and hear (Mt. 13:16; Lk. 10:23).
H.N. Ridderbos, writing the “Kingdom of God” entry in the “New Bible Dictionary”
The announcement of Jesus' kingdom in 1:15 is immediately followed by Jesus calling disciples, driving out demons, healing the sick and forgiving sins. But it is not primarily how Mark uses the phrase kingdom of God. Indeed, Mark uses the phrase much less often than either Luke or Matthew (if Matthew's Semitic use of “kingdom of the heavens” is included), and in a much more restricted sense.
In general, Mark's uses are clumped – of his 14 uses of the phrase, three in chapter 4 and six in 9:47-10:25. It will be instructing to examine them in turn.
In chapter 4, the three uses are the context of Jesus' parables. After telling the parable of the sower, Jesus says to his disciples this enigmatic and much debated saying.
To you has been given the secret of the kingdom of God, but for those outside everything is in parables, so that “they may indeed see but not perceive, and may indeed hear but not understand, lest they should turn and be forgiven.”
He then proceeds to tell two more parables, which he says are explicitly about the kingdom – the parables of the patient farmer and of the mustard seed. All three of the parables have in common the idea of growth within the agricultural metaphor. All three have as a starting point something weak or foolish – the sower in v1-20 sowing seed indiscriminately and much being eaten or choked; the second sower in v26-29 as sowing seed on the ground but doing little in the meantime; the smallness of the mustard seed in v30-32. All have impressive final harvests. The weakness of the starting point fits well in the context of Mark with widespread rejection of Jesus by the establishment and even his own family3 in chapters 1-3. Furthermore, it fits with 4:11-12 – there is a small nucleus of good soil (here the disciples) who will eventually bear spectacular fruit, though not in the course of Mark's gospel.
The next time the phrase occurs is just before the transfiguration, when Jesus says
Truly, I say to you, there are some standing here who will not taste death until they see the kingdom of God after it has come with power.
Mark 9:1, ESV
In Mark, this is at the end of the central transition in the book, from being about who Jesus is to being about the necessity of his suffering and death. Indeed, the transfiguration itself seems somewhat incongruous in Mark – as a glorious interruption into an otherwise largely bleak narrative. It seems wisest to connect Jesus' words to the transfiguration, especially since all three synoptics connect the two. So the transfiguration is an inbreaking of the kingdom – its coming with power to demonstrate there is power in and despite the overwhelming weakness of the kingdom in Mark. It is a hint at the final harvest.
The next two chapter of Mark contain half the references to the kingdom of God in Mark's gospel, and their theme is overwhelming the suffering and death of Jesus and the consequent need for his followers to follow him in the way of the cross, cumulating in Jesus' great saying that
But whoever would be great among you must be your servant, and whoever would be first among you must be slave of all. even the Son of Man came not to be served but to serve, and to give his life as a ransom for many.
Mark 10:43-45, ESV
It is in this context then of the need for suffering, self-sacrificial discipleship in following of the suffering, self-sacrificial Son of Man that we find so many references to the kingdom of God.
9:47 speaks of “entering the kingdom of God”, which is seen as a parallel to “entering life” in v43 & 45, and in opposition to “hell”. The reference to the kingdom is therefore clearly a future eschatological one, stressing the comparative benefits of avoiding sin in the present.
In 10:14-15 , Jesus speaks of the need to receive the kingdom like a little child because it belongs to such as them. The context suggests that the reception of the kingdom is to be done humbly and not holding on to anything else. Again Jesus speaks of “entering it”, which suggests the kingdom is again seen in future eschatological terms.
Much the same could be said of 10:23-25, where the kingdom is referred to three times in the context of it being very difficult for anyone to enter the kingdom, especially the rich. Once again, the notion of sacrificial discipleship suggests itself.
In 12:34, at the end of Jesus' reported conversation with a scribe over the importance of the commands to love God and love neighbour, Jesus says to him “you are not far form the kingdom of God”. The sense is clearly partly commendatory, but also partly as a put-down that the scribe is not in the kingdom of God, which fits perfectly with the progression of relational power dynamics between Jesus and the religious authorities from 11:27 to 12:40. Once again, we get the sense of Jesus saying who is and who isn't in the kingdom – here that understanding and prioritising the law gets this scribe “not far” from the kingdom, but we don't know whether he will enter it or not.
The next occurrence is at the Last Supper, where Jesus says
Truly, I say to you, I will not drink again of the fruit of the vine until that day when I drink it new in the kingdom of God.
Mark 14:25, ESV
At first sight this looks to be another reference to future eschatology, and it is generally understood to be so. However, others have argued that it refers to 15:36, which I shall discuss shortly.
The final reference to the kingdom of God in Mark's gospel is in 15:43, where Joseph of Arimathea is described as “looking for the kingdom of God”, though he seems to be already a follower of Jesus. This again points to a future kingdom, and also hints back to the hidden-ness of the kingdom in 4:11.
These references seem leave the question as to the nature of the kingdom as envisaged by Mark unclear. Is it referring purely to a future event or state, as the second half of the gospel suggests, or is it a growing event which starts with Jesus' ministry and the secrets of which have been revealed to the disciples, as in the first half of the gospel? And what is the transfiguration doing in the middle? Are we then to conclude that Mark did not know what the kingdom was, or indeed that Mark is a composite gospel? We do not need to go so far – I believe that there is another strand in Mark's gospel which will explain the difficulty.
Mark's good news is about Jesus Christ, the Son of God. “Christ” is (trivially) a Messianic title, but the Old Testament understanding of it is to do with kingship – the Messiah is the Davidic king. In the first half of the gospel, the kingdom is hidden, and so is Jesus' identity. But immediately after Peter's confession of Jesus as the Christ at Caesarea Philippi in 8:29, we see that Jesus the Christ is the Son of Man who must suffer and die in 8:31. The kingdom of God having come in power from 9:1 is the proclamation of Jesus as Christ at the transfiguration, but as he is the kind of Christ who must suffer and die, so those who would enter the kingdom must do so too. Just before Jesus enters Jerusalem, he is hailed by Bartimaeus as “Son of David”. As he enters Jerusalem in 11:10 the crowds cry “Blessed is the coming kingdom of our father David”, then come the confrontations with the religious authorities. In 14:62, Jesus tells the priests that he is the Christ, and even does so using the words “I am” with their clear implication of a claim to divinity as well. The expectation is clearly of Jesus' kingship, and that seems to be tied to the notion of the kingdom of God.
The climax comes in chapter 15. Pilate asks Jesus “You are the king of the Jews?” and Jesus replies in the affirmative. After public acclamation for the King of the Jews in 15:13-14, he is crowned by a guard of honour, using royal language, in 15:16-20. Then he is raised up where everyone can see him, with his title - “The King of the Jews” written above his head. The chief priests and the scribes acknowledge him as king in 15:32, he cries to God from a Psalm of the King in 15:34 and then is given wine to drink in 15:36. All of this is done in mockery and with thick irony, but the significance to Mark is clear in the light of what has come earlier. This is the coronation and enthronement of the King who came not to be served but to serve and to give his life as a ransom for many.
To Mark, it seems, that is the kingdom of God. That is when Jesus drinks the fruit of the vine again. That is the mystery that was hidden from the world but revealed to the disciples. That is the seed that would start small, foolish and weak but grow to be larger than all the garden plants. That is the means by which the devil is cast out and overthrown. That is the means by which life is brought to those who would humble themselves to receive it and follow in the way of the cross. That is what the rich simply cannot accept.